Gearing/Speed/RPM Questions 300tdi

A

aeo

Well-known member
1983 110 has a 300tdi (new, and then newly rebuilt)
uprated turbo and uprated intercooler
boost pin
1.2 transmission (newly installed/rebuilt) with 4.11 gearing
31" tires
I live at 5k feet

The feedback I received from the mechanic is that I shouldn't push the RPMs and exceed 2800. I've played with the Ashcroft Gear ratio calculator and know what they say. Right now my top speed seems to be around 60-65 mph on the flats before the RPMs start to climb. Ashcroft looks like it should be 70 mph at 2900 RPM

Questions:
1. What is the realistic highway top end of RPMS for the 300tdi? I am not trying to blow it up and not trying to fly up I-70 but would like to drive from Boulder to Denver at a reasonable pace. I also want to drive to the mountains and want it to go over some decent passes and not get passed by the semi trucks with their hazards on.
2. What are realistic speed expectations at altitude?
3. Larger tires or different rear gearing are another option that the mechanic has brought up to fix my top end speed. How big of a tire can I run with a OME suspension? It's fairly close to stock so maybe 1-2" lift.

It seems like a decade later I am so close to a decent running truck.
 
Napalm00

Napalm00

Technical Excellence Contributor
Climbing passes at altitude even with a tuned truck you may be down in the 45mph range
 
JimC

JimC

Super Moderator
Staff member
you’ve got a lot of non-stock variables there, why don’t you get an EGT gauge (pyrometer) and monitor your temps? An arbitrary RPM limit is a pretty poor metric. Monitor temps during easy drives (probably won’t be more than 750-850) and back off when you start to push 11-1200. You’ll also notice fuel consumption increases dramatically as you push higher temps.
 
A

aeo

Well-known member
Thanks javelindave. I had to get a real job to off set my Rover parts...

JimC- I have an EGT. I’ll use that instead. I have a Madman and typically have the EGT on display and watch that. Has a boost gauge too.
 
RDavisinVA

RDavisinVA

Technical Excellence Contributor
Between the altitudes and upgrades, the answer is you'll be traveling dangerously slow.
 
Napalm00

Napalm00

Technical Excellence Contributor
I'm still not understanding it guys. If I calculate out this she's actually geared probably more correctly than I am l.

I'm at early lt77, 3.54, 35in tires and a 1.4. but only puts me about a hundred RPM lower than her in every gear. What's the issue ?

The numbers that she gives in terms of top speed sound perfectly reasonable, am I missing something?


Fixed
 
Last edited:
A

aeo

Well-known member
It is close to stock- 3.54 vs 4.1. When we first stuffed a 300tdi in the old girl she had a 1.4 and 4.1 gears. This last motor conversion we did a 1.2 but haven’t touched the rear gears. Mechanic suggested the 1.2 because it was really revving high on the rpms to get a decent top end speed around 6- mph. EGT is decent around town stays around 500-700 unless I try to drive it like a v8.

So- what is the best plan to avoid traveling dangerously slow uphill- different rear gears or bigger tires?
 
Napalm00

Napalm00

Technical Excellence Contributor
Neither really . You are at what most would consider close to optimum performance on a TDI.

You need more power not better gearing.

So tune, injectors, air filter, turbo , intercooler, flowed head, boost controller ect . You may be able to eek out another 5-10 mph with similar egts if it's tuned on the nose.


Or engine swap
 
R

Red90

Well-known member
Don't worry about the engine speed. You can run at 3500 all day if you want. It won't hurt anything. With an uprated intercooler and turbo, you should be able to do 85 and comfortably cruise at 75.

You may need some adjustments on boost and fueling. If you want some advice, let us know the boost level and the EGT at sustained full throttle.

5000 feet should not have any effect on power.
 
A

aeo

Well-known member
Red90 said:
Don't worry about the engine speed. You can run at 3500 all day if you want. It won't hurt anything. With an uprated intercooler and turbo, you should be able to do 85 and comfortably cruise at 75.

You may need some adjustments on boost and fueling. If you want some advice, let us know the boost level and the EGT at sustained full throttle.

5000 feet should not have any effect on power.
Click to expand...


Ran up a 6% hill for 1.5 miles. 45 mph, EGT went up to 1000, boost was pegged 14- that's the end of the gauge. Let off and seemed to hold the 40-45 mph speed. Beginning to think it is more user error or inappropriate expectations than anything with the motor.

I'd love to get it up to Steamboat Springs- I think it would be happy in town- not a lot of elevation gain, low speeds in town, off road driving. But I fear the drive up here.
 
Napalm00

Napalm00

Technical Excellence Contributor
You're leaving a lot of power on the table by keeping boost at 14 and backing off at 1000f.

Pump up the boost and push it harder !
 
Napalm00

Napalm00

Technical Excellence Contributor
This is the one I prefer for wg turbo and VNT if you are using a boost actuated VNT like the bell/allisport/Allard .

www.grimmspeed.com

GrimmSpeed Manual Boost Controller - Black

THE NEXT LEVEL OF BOOST CONTROL Welcome to the most refined manual boost controller available on the market today. During development, we worked hand in hand with tuners across the country to develop this device and ensure that it solves legitimate problems that they have with other existing...
www.grimmspeed.com www.grimmspeed.com

If you are using any other VNT tht has a vacuum actuator there is another style.
 
  • Like
Reactions: Z.G
You must log in or register to reply here.
Top